To facilitate the study of the cellular and molecular mechanisms of T cell-mediated regulation of B cell differentiation, we will use the DNP-specific IgA-secreting BALB/c mouse myeloma MOPC-315 as a monoclonal model of B cell differentiation. MOPC-315 cells differentiate from small, non-secretory lymphocytes (which predominate during early tumor growth) to large IgA315-secreting plasmacytes during growth in normal BALB/c mice. That differentiation is sensitive to regulation by antigen-specific and idiotype-specific regulatory T lymphocytes. By centrifugal elutriation, MOPC-315 cell subpopulations will be separated by size and their functional activities (IgA secretion, stem cell activity, etc.) will be determined. Using these homogeneous MOPC-315 subpopulations, those surface markers and metabolic activities which are specific for each stage of B cell differentiation will be determined. I will also monitor how exposure of these neoplastic B cells to T cell regulatory factors and/or antigen affects the expression of the differentiation stage-specific markers. MOPC-315:MOPC-104E hybridomas which produce and secrete both IgA315 and IgM104E will be produced and used to probe how exposure to one regulatory stimulus affects the B cell's ability to respond to the opposite or same regulatory signal given later. These experiments will allow us to begin to understand how T cell regulatory signals are translated by the B lymphocyte.